Question

Discuss your observations regarding the acid/base chemistry of the Mn(IV) dimer, including stoichiometry, reversibility, and temperature effects.

Protonation of [MnIV(salpn)(µ-O)]2omplex gets protonated? Where else could you have protonated the complex

• Dissolve 0.1 mmol (70 mg) of [MnIV(salpn)(O)]2 in 200 mL of methylene chloride. Divide the solution into 4 equal portions. You may want to reserve a small amount of this solution (approx. 10 mL) for color comparison. For each of the following steps, be sure to make careful observations regarding the color of the solution. The best way to do this is to bring a little bit up in a pasteur pipette and use a flashlight (you may use your phone for this purpose) to aid in seeing the color of the solution.

• To one portion, add one equivalent of triflic acid (25 µL of 1.0 M solution in acetonitrile).

• To a second, add two equivalents triflic acid (50 µL of 1.0 M solution).

• Cool the two remaining portions to –78 °C in a dry ice/acetone bath, and add one equivalent triflic acid to one portion and two equivalents to the other. (Keep the flasks covered while cooling to avoid water condensation in the solutions.)

• Add 1 equivalent of triethylamine (7.2 M, so .025 mmol = 3.5 µL: For convenience, dilute 100 µL to 1 mL in acetonitrile and use 35 µL per equivalent) for each equivalent of triflic acid added to each solution above

Answer 1

Addition of 1 equivalent of a strong acid (triflic acid) to [Mn^IV(salpn)(µ-O)]₂, generates the oxo/hydroxo complex [{Mn^IV(salpn)}₂(µ-O,µ-OH)](CF₃SO₃), which emphasize the basicity of the µ2-O2-units in the [Mn^IV( µ-O)]₂ dimers. This protonation significantly affects the dimer by: inhibiting the catalese like activity, increasing the Mn-Mn distance, lowering the energy of lowest charge transfer band, with a change in color from red-brown to deep purple.

Addition of 2 equivalents of triflic acid causes further reaction marked by loss of the deep red-brown color of [Mn^IV(salpn)(µ-O)]₂ , resulting in a light yellow-brown Mn^III product.

At -78 ^oC, this decomposition reaction does not occur and the doubly protonated complex [Mn^IV(salpn)(µ-OH)]₂(CF₃SO₃)₂ is formed. Both the first and second protonation are reversible, and [Mn^IV(salpn)(µ-O)]₂ is regenerated with stoichiometric addition of base.

The complex [Mn^IV(salpn)(µ-OH)]₂(CF₃SO₃)₂ can be isolated as a solid, and dissolved in cold acetonitrile.